Myotonic dystrophy type 1 (DM1), which is caused by CTG expansions (CTGexp) in the 3' untranslated region of the DMPK gene, has been used as a model for RNA-mediated disease mechanisms associated with other microsatellite expansion diseases, including fragile X tremor/ataxia syndrome (FXTAS) and C9orf72 amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). In DM1, transcription of the CTGexp mutation results in CUGexp RNAs that alter the developmental regulation of pre-mRNA processing and mRNA localization events mediated by the MBNL and CELF families of RNA binding proteins. However, additional cellular pathways, such as miRNA processing and repeat-associated non-AUG translation, have also been implicated in DM1 pathogenesis. Most importantly, no effective therapies exist to treat this neuromuscular disease. To address these deficiencies, this project is designed to generate more informative mouse experimental models for DM1 to elucidate the relative contribution of each of the proposed pathomechanisms and qualify RNA splicing defects as responsive biomarkers of therapeutic response with the goal of developing effective therapeutic approaches to decrease the toxic burden of CUGexp RNAs. Aim 1 builds upon our recent development of Dmpk CTGexp knockin mice generated using a combination of rolling circle amplification to generate large repeats in vitro and CRISPR/Cas9-mediated genome modification. Using an allelic series of increasing CTG repeat lengths that represent the late-onset to congenital spectrum of the DM1 pathogenic range, we will determine CTG length-dependent effects on skleletal and heart muscle structure/function, RNA processing/localization/turnover and RAN translation. Transcriptome analysis will be pursued further in Aim 2, which is based upon our prior observations that patient functional impairment corresponds to RNA splicing defects and MBNL loss of function, to determine if splicing defects qualify as effective biomarkers that are responsive to CUGexp levels, MBNL activity and therapeutic intervention. In Aim 3, we will broaden this therapeutic scope and evaluate multiple strategies, including antisense oligonucleotide (ASO)-mediated CUGexp knockdown and small molecule approaches to inhibit transcription of mutant Dmpk CTGexp genes. The overall objective of this project is to provide the DM field with more robust mouse models of DM1 while also evaluating splicing defects as biomarkers of disease status and developing single small molecule strategies